Bandwidth Quotas (NOT QoS)

I am genuinely curious about why almost no firmwares support hard bandwidth quotas. Gargoyle is the only firmware that I know of which does support it. QoS is the wrong tool for the job; being an excellent tool for managing the flow of data so that no one user hogs the bandwidth from others, it is essentially useless in the face of hard data caps (which are becoming more common by the day). To make an example; an ISP with a 250 GB monthly cap and 10/5 Mbps would require that the maximum combined upload be limited to about 248 kbps and the maximum combined total download speed be limited to about 500 kbps. If any faster speeds are allowed, it would be possible for a 24x7 client to exceed the monthly cap. As you can imagine, just have two or three people on a connection limited to 500 kbps is going to suck - even Youtube at 240p isn't going to run without constant sputtering.

Why does almost every router firmware in existence leave users with no realistic way of insuring that they are living within their bandwidth caps? Even my Android phone has this capability. I'm assuming that it must be very difficult to implement, or that there's a patent on the technology.